1. Technical Field
The present invention relates generally to the field of inverter circuits, and in particular to inverter circuits using pulse width modulation techniques (PWM) to approximate desired output waveform characteristics.
2. Background Art
In a prior-art technique known as "sine-coded PWM," a control circuit causes the three outputs of a three-phase inverter to approximate the voltages of three corresponding modulating sinusoidal waveform inputs by adjusting the respective duty cycles of each of the three outputs. The control circuit accomplishes this by comparing each input waveform with a high-frequency triangular carrier waveform. Switching means corresponding to each inverter output are switched on or off depending upon whether the corresponding modulating waveform input is above or below the carrier waveform. The basic sine-coded PWM technique reaches a limit as the amplitude of a modulating waveform approaches that of the carrier wave. When the amplitude of the modulating waveform exceeds that of the carrier wave, an undesirable phenomenon known as "pulse-dropping" occurs.
In one prior-art solution to the problem of pulse-dropping, a third harmonic waveform is injected into the modulating sine waveform. The injection of the third harmonic allows the fundamental component of the modulating waveform to be higher before pulse dropping occurs. This approach is disadvantageous, however, because of the greater complexity of the modulating waveform.